Another silly simple precision 0/20mA to 4/20mA converter

A recent Design Idea (DI), “Silly simple precision 0/20mA to 4/20mA converter,” by prolific DI contributor Stephen Woodward uses the venerable LM337 regulator in a creative configuration along with a few passive components, to translate an input 0-20 mA current source (say from a sensor with a separate power source that outputs a 0-20 mA signal current) into a 4-20 mA two-wire transmitter current loop (a standard 2 terminal industrial current source).

Below is another novel, ‘silly simple’ way of implementing the same function using the LM337. It relies on tapering off an initial 4 mA current to zero in proportion to the input 0-20 mA, and adding the input and the tapered off 4mA signal to create a 2-wire 4-20 mA output loop. It is loosely based on another Woodward gem [3]. Refer to Figure 1.

Figure 1 An input 0-20 mA is added to a tapered-off 4-0 mA at OUT to give an output 4-20 mA.

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First, imagine ‘0 mA’ current input (input loop open). The series arrangement of R1 parallel ‘R2 + Pz’ (‘Rz’@250E) and R3 parallel ‘R4+Ps’ (‘Rs’@62.5E) having a nominal value of 312.5E, sets the value of output loop current into OUT at 0mA+4mA (1.25V/312.5E), set using Pz.

Now, feeding a 20mA input current, imagine it pulled from junction X and pushed into the OUT terminal. This current is sourced from the output loop ‘+’, dropping 62.5E x 20mA=1.25V in Rs, in a direction opposing the internal reference voltage. With proper calibration, this reduces the drop across Rz to zero, and in doing so, reduces the original 4 mA contribution through Rz into OUT, also to zero.

The output loop current is now equal to the input current of 20mA+0mA (added at OUT), transferred from the input loop to the output loop from OUT to IN of U1. We have converted a current source input of 0-20 mA to a 2-wire loop current of 4-20 mA. The 20 mA setting is done by Ps.

Accurate current setting requires 2 S/Z passes to set the output current to within 0.05% or (much) better. Pots should be multi turn 3296 types or similar, but single turn trimmers will also work fairly well as both pots have a small trim range, by design.

The performance is excellent. The input to output linearity of the basic circuit is 0.02%. With a small heat sink, short term stability is within 0.02%, and change in loop current is 0.05% over a voltage from 5 V to 32 V. Transfer accuracy and stability are high because we aren’t transforming the input signal, only transferring it into the output loop. Reference drift affects only the basic 4 mA current and thus has a smaller effect on overall drift. The heat sink improves drift and di/dv by a factor of 3 to 4.

For intermediate input currents, the 4mA basic current via Rz into OUT is tapered off in proportion to the input 0-20 mA current. Thus at 10 mA (half) input current, the voltage at X changes suitably to maintain @500 mV across Rz, this supporting a contribution of 2 mA into OUT, down from the original 4 mA set at 0 mA input current. Output loop current into OUT is now the input 10mA+2mA=12mA, the halfway point of the 4-20 mA loop too. Similar reasoning applies to other input/output loop currents relationships.

A reverse protection diode is recommended in the 4-20 mA loop. Current limiting should be applied to limit fault current to safe levels. A series 2-transistor current limiter with appropriate resistance values is an excellent candidate, being low drop, low cost, fast acting and free from oscillation. A 40-mA ptc ‘polyfuse’ in the loop will protect the load from a complete short across both circuits (an unlikely event).

The basic drop seen by the 0-20 mA signal is -1 V to 0 V. Two diodes or an LED in series with the + of the 0-20-mA input allow the source to always see a positive drop.

Regarding stability: only the 68E(R3) and the 270E(R1) need to be 25 ppm 1% types to give low overall temperature drift, which is a significant plus. Pot drift, typically larger than that of fixed resistors, has less effect in the configuration used, wherein pots Ps and Pz, relatively high valued, control only a small part of the main current. Larger pot values also help minimize the effect of varying pot contact resistance.

A 3-V minimum operating voltage allows as much as 1000E of loop resistance with a 24-V supply, for the basic circuit.

It is a given that one of the loops will (need to) be floating. This is usually the source loop, as the instrument generating the 0-20 mA is powered from a separate supply.

Ashutosh Sapre lives and works in a large city in western India. Drifting uninspired through an EE degree way back in the late nineteen eighties, he was lucky enough to stumble across and be electrified by the Art of Electronics 1 and 2. Cut to now, he is a confirmed circuit addict, running a business designing, manufacturing and selling industrial signal processing modules. He is proud of his many dozens of design pads consisting mostly of crossed out design ideas.

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